As shown in FIG. 1, a nuclear power station conventionally includes a reactor pressure vessel 10 with various configurations of fuel and reactor internals for producing nuclear power. For example, vessel 10 may include a core shroud 30 surrounding a nuclear fuel core 35 that houses fuel structures, such as fuel assemblies, 40. A top guide 45 and a fuel support 70 may support each fuel assembly 40. An annular downcomer region 25 may be formed between core shroud 30 and vessel 10, through which fluid coolant and moderator flows into the core lower plenum 55. For example, in US Light Water Reactor types, the fluid may be purified water, while in natural uranium type reactors, the fluid may be purified heavy water. In gas-cooled reactors, the fluid coolant may be a gas such as helium, with moderation provided by other structures. The fluid may flow upward from core lower plenum 55 through core 35. In a water-based reactor, a mixture of water and steam exits nuclear fuel core 35 and enters core upper plenum 60 under shroud head 65. One or more control rod drives 1 may be positioned below vessel 10 and connect to control rod blades or other control elements that extend among fuel assemblies 40 within core 35.
Nuclear reactors are refueled periodically with new fuel to support power operations throughout an operating cycle. During shutdown for refueling, the vessel 10 is cooled, depressurized, and opened by removing upper head 95 at flange 90. With access to the reactor internals, some of fuel bundle assemblies 40 are replaced and/or moved within core 35, and maintenance on other internal structures and external structures like control rod drive (CRD) 1 may be performed from outside of reactor 10.
As shown in FIG. 2, CRD 1 may be mounted vertically within a CRD housing welded to a stub tube 8, which may extend up into reactor pressure vessel 10. A spud 46 at a top of index tube 26 may engage and lock into a socket at the bottom of the control element, and index tube 26 may vertically move through action of the CRD hydraulic system to vertically drive or hold the control element. CRD 1 and any control rod element connected via spud 46 form an integral unit that is manually uncoupled by before CRD 1 or control element may be removed from reactor 10. Below vessel 10, in an access area or drywell, CRD flange 6 may extend downward from vessel 10 and the CRD housing. CRD 1 may be secured to a face by mounting bolts 88 in flange 6. A pressure-tight seal can be created by O-ring gaskets (not shown) between flange 6 and any mounting surface.
One or more CRD hydraulic system lines 81 may pass through ports in flange 6 and work with a CRD hydraulic system for CRD operation, inserting, holding, and/or withdrawing a control element (not shown) via spud 46 at desired positions and speeds for reactor operation. For example, CRD flange 6 may include a withdraw port 82 and an insert port 83 with a check valve 20. Lines 81 may carry water to insert port 83 and from withdraw port 82. Withdraw port 82 may serve as an inlet port for water during control rod withdrawal, via vertical downward movement of spud 46. A piston port 69 may connect to withdraw port 82 in CRD flange 6. Through piston port 69, water and hydraulic pressure may be applied through an under-the-collet-piston annulus to collet piston 29 to cause withdrawal, or downward vertical movement of spud 46. For normal or scram insertion, via vertical upward movement of spud 46, water may be supplied to inlet port 82, and withdrawal port 82 may work as an outlet port for water. For rapid shutdown, such as scram insertion with rapid upward movement of spud 46, check valve 20 may direct external hydraulic pressure or reactor pressure to an underside of drive piston 24.
FIG. 3 is a detail view of a bottom of flange 6, showing an area for insertion of probe 12a. As shown in FIG. 3, piston tube 15 extends upward through the length of CRD 1, terminating in a watertight cap near the upper end of the tube section and, oppositely, at a threaded end secured by a fixed piston tube nut 16 at the lower end of CRD 1. A position indicator probe 12a may be slid into piston tube 15 from the bottom, potentially sealed into indicator tube within the same. External to piston tube 15, probe 12a can be welded to a plate 12b bolted to housing 12 extending from a bottom of flange 6. Housing 12 may be secured to CRD ring flange 17, a downward extension of flange 6, by housing screws 13. In turn, ring flange 17 may be secured to flange 6 by flange screws 9. Probe 12a and housing 12 attached about fixed piston tube nut 16 as a unit, removable from a bottom of flange 6 together through removal of housing screws 13.
Probe 12a transmits electrical signals to provide remote indications of control rod position and CRD operating temperature. Probe 12a can include a switch support with reed switches and a thermocouple for transmitting electrical signals to provide remote indications of control rod position and CRD operating temperature. The reed switches are normally open but may be closed individually during CRD operation by a ring magnet in the bottom of drive piston 24 (FIG. 2). The reed switches are connected by electrical wires to a connection port 14 that may extend outside of housing 12 and provide wired connectivity to remote operators. Housing 12 may protect any electrical wires extending into connection port 14.
In order to uncouple a control element from CRD 1, a lock plug in spud 46 (FIG. 2) may be raised from below by operators working below reactor vessel 10. Conventionally, position indicator probe 12a is removed from CRD 1 prior to drive removal to allow access to piston tube 15 by an uncoupling tool. Operators typically manually attach an uncoupling tool is to a bottom of CRD 1 and apply force, such as with a jack, to raise piston tube 15. When the control element is in its “full-out” position directly atop stub tube 8, drive piston 24 may be separated from a piston head by a small distance. Operators typically observe this positioning directly under vessel 10, and when the positioning is reached, give an indication for removal by other operators. Raising piston tube 15 by this distance lifts the lock plug out of spud 46, allowing spud and piston together to be withdrawn and disengage from a control element.